A cloth dryer having a built-in heat pump, which allows effective use of heat, has been proposed recently (disclosed in e.g. Patent Literature 1). The heat pump is formed of the following structural elements:
a compressor for compressing a refrigerant;
a heat radiator for exchanging heat between the refrigerant, which has been compressed by the compressor and turned into a high temperature and high pressure state, and the ambient air, thereby radiating the heat from the refrigerant;
a throttling section for decompressing the highly pressurized refrigerant having undergone the heat radiator;
a heat absorber for exchanging heat between the refrigerant, which has been decompressed by the throttling section and turned into a low pressure and low temperature state, and the ambient air, thereby depriving the ambient air of the heat; and
a pipe line for the refrigerant to travel through the foregoing structural elements one by one.
The cloth dryer including the foregoing heat pump works this way: Drying air blown by a blower deprives clothes placed in a rotary drum of water, so that the air becomes humid. Then the blower transmits the air to the heat absorber of the heat pump through a circulating duct. The drying air of which heat is deprived by the heat absorber is dehumidified and conveyed to the heat radiator to be heated, and then circulated into the rotary drum again. The drying air repeats the foregoing steps, whereby the clothes are dried.
The structure disclosed in Patent Literature 1 allows the water vaporized from the clothes to form dew on the heat absorber, so that the clothes can be dried efficiently. On top of that, heat of hot wind containing the water from the clothes is absorbed by the heat absorber, and the heat is transmitted to the compressor via the refrigerant, which is heated by the compressor, and the heat of the refrigerant is radiated by the heat radiator for heating again the hot wind. The heat can be thus efficiently used.
The dryer using the heat pump disclosed in Patent Literature 1 allows the heat absorber to dehumidify the dumped clothes, so that the heat absorber can work as a heat absorbing source of a refrigerating cycle. Electric power is input for circulating the refrigerant, so that the heat radiator can heat the air for further vaporizing the water from the clothes. The foregoing steps are repeated.
However, the conventional cloth dryer using the heat pump discussed above takes a time before the clothes are warmed and ready for being used as the heat absorbing source of the refrigerating cycle, and the compressor resists increasing a pressure before the heat absorbing source is ready.
When the clothes are in a low temperature state or the cloth dryer per se is in a low temperature state because an ambient temperature is low, e.g. in winter, the air circulating through the heat absorber and the heat radiator, which form the refrigerating cycle, falls into a low temperature state. In such a case, the refrigerant flowing in the heat absorber should be controlled at a temperature lower than the temperature of this air in order to carry out the heat exchange between the refrigerant and the air, otherwise, the refrigerant cannot absorb the heat from the air.
The refrigerant flowing in the heat absorber thus remains not higher than 0° C. until the temperature of the circulating air rises to a given temperature. The water forms dew on the heat absorber and grows to frost or ice, which attaches to the surface of the heat absorber. As a result, the frost or ice attached to the surface blocks the circulating air and also disturbs the heat exchange between the refrigerant and the air.
In the heat absorber, the air is cooled greater as the air runs further down the flow, so that the temperature at the downstream becomes the lowest. The frost or ice thus starts growing from the downstream and blocks the circulating air, and also disturbs the heat exchange between the refrigerant and the air.
The frost or ice repeats growth and meltdown on the surface of the heat absorber until the circulating air is warmed to a given temperature. The water melted down drops to the underside of the heat absorber and is frozen again. The re-frozen ice-layer on the heat absorber blocks the circulating air and also disturbs the heat exchange between the refrigerant and the air.
On top of that, when the heat exchange between the refrigerant and the air is carried out unsatisfactorily due to the growth of frost or ice on the heat absorber, the refrigerant cannot fully evaporate and is sucked into the compressor in a liquid state. This phenomenon will affect the reliability of the compressor.
Patent Literature 2 discloses another structure of the heat pump used as a heat exchanger for a dehumidifier. A heat absorber and a heat radiator of this heat pump share fins and form a heat exchanger in one body, and slits are provided at the fins between the absorber and the radiator. This slit allows suppressing the flow of heat between the absorber and the radiator, so that they can be downsized.
However, in the heat exchanger disclosed in Patent Literature 2, pipe-lines for the refrigerant at the absorber and the radiator share the fin and the pipe-lines are adjacent to each other. The absorber and the radiator thus invite heat transfer through the fins between the adjacent pipe-lines, so that the efficiency of the heat exchange is lowered.
On top of that, when the air traveling through the heat exchanger is at a high temperature, the heat transfer discussed above makes it difficult for the heat radiator to maintain a refrigerant overcooled region, so that the dehumidifying capacity is lowered.
Another heat exchanger for an air-conditioner or a refrigerator is disclosed in, e.g. Patent Literature 3. In this heat exchanger, a rather longer cut section is provided at the following two places respectively: at a heat transfer pipe where a refrigerant enters and a rather higher temperature is kept, and at another heat transfer pipe where the refrigerant exits and a rather lower temperature is kept. This structure allows cutting off efficiently the heat conduction between the heat transfer pipes where temperatures different greatly from each other are kept, so that a greater refrigerant overcooled region can be obtained. As a result, a greater amount of heat exchange, i.e. a greater capacity of heat exchange, can be expected.
The heat exchanger disclosed in Patent Literature 3; however, in a case where multiple rows of refrigerant pipes exist between the entrance and the exit for the refrigerant, heat transfer occurs through the fins between the adjacent refrigerant pipes. The foregoing structure thus incurs degradation in the efficiency of maintaining a high temperature at the heat radiator, or degradation in the efficiency of maintaining a low temperature at the heat absorber. As a result, no further improvement in the efficiency can be expected regrettably.    Patent Literature 1: Unexamined Japanese Patent Application Publication No. H07-178289    Patent Literature 2: Unexamined Japanese Patent Application Publication No. 2002-310584    Patent Literature 3: Granted Japanese Patent Publication No. 3769085